This invention relates to the field of current measurement, particularly in the context of pulse-width-modulated (PWM) circuits. More specifically, the invention includes a circuit which provides an instantaneous and accurate measurement of current flowing through a load, while maintaining galvanic isolation between the measurement circuit and the load.
Examples of PWM circuits are shown in U.S. Pat. Nos. 5,070,292, 5,081,409, 5,379,209, and 5,365,422. The disclosures of all of the latter patents are hereby incorporated by reference into this specification. These patents give examples of circuits in which a series of pulses is used to control electronic switches which selectively connect a power supply to a load. The load can be an electric motor, or a coil used to produce a magnetic field, or some other load.
In PWM circuits of the types described in the above-cited patents, it is often necessary to monitor the current flowing through the load, either for purposes of overcurrent protection, or because one wants to control another circuit based on the measured current in the load, or for other reasons. Direct measurement of load current is undesirable because it requires that one insert an inductance or a resistance into the circuit being measured. The preferred means of current measurement is one which maintains galvanic isolation, i.e. insuring that no current flows directly between the load and the measuring circuit.
However, in the prior art, there are few techniques for measuring load current in a PWM circuit while maintaining galvanic isolation. While one could simply couple the load, through a transformer, to a conventional circuit for current measurement, the accumulation of magnetic flux in the transformer core accentuates the nonlinearity of the transformer and introduces inaccuracy into the final measurement. A solution to this problem is to use a larger transformer, because the larger the transformer, the less likely that the transformer core will become saturated, and the greater the range over which the transformer response is relatively linear. But using a larger transformer has the disadvantage of requiring a larger space, and it may also be unacceptably expensive.
The present invention provides accurate measurement of the current in a load, in a PWM circuit, using components that are both readily available and relatively inexpensive. The invention provides means for effectively maintaining the flux density in the transformer core at or near zero, so that distortion in the measurement is minimized.